A copolymer of diallyldialkyl ammonium salt and maleic acid is obtained by employing diallyldialkyl ammonium salt and maleic acid as monomers and copolymerizing them in an aqueous solution (see Patent Document 1), and it attracts attention regarding its various uses such as a flocculating agent, a deodorant, a paper making auxiliary, etc. (see Patent Document 2).
According to studies made by the present inventors, when monomers for the above copolymer are polymerized under conditions for producing a low molecular weight product of the polymer, there are some cases where the polymerization yield is low and residual monomers exist. Further, when an attempt is made to produce a terpolymer of diallyldialkyl ammonium salt, maleic acid and sulfur dioxide, it is required to carry out polymerization at a relatively low temperature, so that there are also some cases where the polymerization yield is low and residual monomers exist. As a result, when these are used in various uses, the residual monomers sometimes affect the performance thereof.
Meanwhile, it is known to use a distillation method or an ion-exchange membrane electrodialysis method for the removal of cationic monomer, etc., that are impurities after the polymerization. According to studies made by the present inventors, when the cationic monomer is a diallyldialkyl ammonium salt, there has been a problem that it is difficult to remove the cationic monomer as compared with normal monomers since it is a non-volatile quaternary salt. It has been hence studied to decrease the residual monomer by using a large amount of a polymerization initiator. However, it has been found that a very small amount of a remaining polymerization initiator sometimes affects the performance in various uses to cause a problem anew.